(1) Technical Field
This invention relates generally to an apparatus and method for making integrated circuits and more particularly, the invention relates to methods that prevent die loss resulting from planarization using chemical mechanical polishing.
(2) Description of the Prior Art
The following four documents relate to methods dealing with chemical mechanical polishing of integrated circuits formed on semiconductor wafers.
U.S. Pat. No. 5,271,798 issued Dec. 21, 1993 to G. Sandhu et al., remedies the problems associated with selective etching of material, in particular tungsten, by locally removing the material, e.g. tungsten, from the alignment marks through wet etching without the need for any photo steps. Either before or after chemical mechanical polishing, the wafers are flatly aligned and a tungsten etching agent is introduced through an etching dispensing apparatus onto the alignment marks.
U.S. Pat. No. 5,362,669 issued Nov. 8, 1994 to J. Boyd et al., discloses a method of chemical mechanical polishing using polish stops in wide trenches.
U.S. Pat. No. 5,401,691 issued Mar. 28, 1995 to Caldwell, shows a method of forming an alignment mark using an inverse open frame.
U.S. Pat. No. 5,627,110 issued Lee et al. shows a method for eliminating a window mask process in the fabrication of a semiconductor wafer when chemical mechanical polish planarization is used.
The fabrication of integrated circuits on a semiconductor wafer involves a number of steps where patterns are transferred from photolithographic photomasks onto the wafer. The photomasking processing steps opens selected areas to be exposed on the wafer for subsequent processes such as inclusion of impurities, oxidation, or etching.
During the forming of integrated circuit structures, it has become increasingly important to provide structures having multiple metallization layers due to the continuing miniaturization of the circuit elements in the structure. Each of the metal layers is typically separated from another metal layer by an insulation layer, such as an oxide layer. To enhance the quality of an overlying metallization layer, one without discontinuities of other blemishes, it is imperative to provide an underlying surface for the metallization layer that is ideally planar. The process of planarizing is now a standard process application of integrated circuit manufacturers.
Plasma, or reactive ion etching of the oxide layers having a resist planarizing medium, are conventional planarization techniques that are used to provide a smooth surface and a local planarization with a range of 1 um.
To meet the demand for larger scale integration, and more metal and oxide layers in devices and the exacting depth of focus needed for submicron lithography, a new planarization method, known as chemical mechanical polishing (CMP), was developed and is presently used by most major semiconductor manufacturers. CMP planarization of a wafer involves supporting and holding the wafer against a rotating polishing pad wet with a polishing slurry and at the same time applying pressure. Unlike the conventional planarization techniques, CMP provides a substantially improved overall planarization, that is, an improvement of 2 to 3 orders of magnitude over conventional methods. Although CMP planarization is effective, die loss still occurs. Referring now to FIGS. 1a and 1b, the problem associated with present CMP planarization is a "high plateau" effect, surrounding alignment targets 11, 12 and the outside periphery 21 of the die array 20 which can cause a non-uniform polishing of the substrate surface. Stepping fields 15, 16 and 21 are skipped during alignment and exposure of substrate 10, the photoresist layers over blank stepping fields 15, 16 and 21 are never exposed during the process. Thus, all layers deposited on substrate 10 are always deposited over blank stepping fields 15, 16 and 21 but are never removed or etched away. The blank stepping fields 15, 16 and 21, therefore, consists of the worse case (highest) elevation structure on the wafer.
As noted, the build up of the deposited materials on the blank stepping fields 15, 16 and 21 appears to the polisher as a high plateau area. Since the area is large and flat, the polish rate of blank stepping fields 15, 16 and 21 is slow, like that of a blank wafer. The high plateau of the blank stepping fields lowers the polish rate of the edges of the IC die 20 which immediately surround blank stepping field 15, 16 and the wafers edge 21. The high plateau effect, therefore, can cause an unacceptable yield falloff on die which border blank stepping fields.
As a result, the prior art technique has difficulty in providing trouble free device patterns causing open contact or via and thus, causes a problem that the process margin in fabricating semiconductor devices is reduced, so as to lower production yield.